If you haven't played Diablo III yet, then the upcoming home console version wouldn't be a bad place to start. Blizzard has been working hard at making sure the game is easily playable for those with a joypad - and this new trailer demonstrates how co-op multiplayer works on the PS3.
The video does a good job of highlighting just how much hell a party of four can raise in the game. Looks quite simple to jump into a game, and we would imagine that the difficulty curve runs up depending on the number of players in the world at any one time.